An enzymatic model of the growth hormone-releasing hormone oscillator incorporating neuronal synchronization

Models of growth hormone (GH) rhythmogenesis which we and others have presented suggest that the GH pulses in the circulation are generated by a GH-releasing hormone (GHRH) oscillator with a 1 h periodicity. Here we examine the possibility that this is an intrinsic neuronal rhythm resulting from enzymatic reactions occurring in the axon terminals. A "Baselator" feedback reaction sequence can generate an hourly chemical burst of a primer (presumably a low molecular weight peptide) regulating Ca2+-triggered exocytosis of GHRH-loaded vesicles. Accordingly we propose that the priming species is largely immobilized by binding within the terminals. Free unbound primer is able to diffuse and is alternately phosphorylated and dephosphorylated by a kinase and a phosphatase (or undergoes a similar pair of complementary reactions). Under appropriate conditions involving feedback control of one or other of the enzymes the levels of both unreacted and reacted free primer peak sharply at hourly intervals.It is self-evident that synchronization between the packed terminals of the GHRH neurons at the median eminence is necessary to generate highly ordered in vivo pulses of GH release. Gap junctions provide a means of interterminal communication for the primer. Simulations of clusters of 4 adjacent axon terminals in a linear array were performed to assess whether and when synchrony can occur. With gap junctions closed the axons were set to be 90° out of phase, i.e. their chemical bursts were separated by 15 min. Opening the gap junctions, assuming either that only the unphosphorylated species permeates, or that both species permeate, resulted in rapid overall synchronization. The oscillatory systems undergo mutual entrainment and all peaks appeared simultaneously at an intermediate hourly interval. This result was independent of the mode of chemical feedback, whether positive or negative. Closing the gap junctions led to a gradual, but not immediate, loss of synchrony.